package com.xywl;

import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        Scanner sc=new Scanner(System.in);
        double x1=sc.nextDouble(),y1=sc.nextDouble();
        double x2=sc.nextDouble(),y2=sc.nextDouble();
        double dx=x1-x2;
        double dy=y1-y2;

        System.out.printf("%.4f\n",Math.sqrt(dx*dx+dy*dy));
    }
} 